The invention relates to regulating the flow of fuel delivered to a turbomachine combustion chamber. The invention is applicable to all types of turbomachine, and in particular to aviation turbines and to industrial stationary turbines.
FIG. 1 shows, in highly diagrammatic manner, a well-known conventional flow rate regulator device for use, for example, in an airplane turbomachine.
A low-pressure fuel circuit 1 supplies a high-pressure pump 2 with fuel taken from a tank by means of a low-pressure pump. The outlet from the high-pressure pump 2 is connected to a metering unit 3 having a slide of position that is controlled by an electrohydraulic servo-valve 3a in order to adjust the flow section through the metering unit. A bypass valve 4, e.g. rated by a spring, has ports connected respectively to the inlet and to the outlet of the metering unit 3 in order to maintain a constant pressure difference dP between said inlet and outlet so that the flow rate through the metering unit is a function of its flow section. The bypass valve 4 is mounted in a return loop 5 for returning to the low-pressure fuel circuit 1 the fuel that is delivered in excess by the high-pressure pump, since its speed is associated with the speed of the turbomachine.
An electronic control unit 6, generally incorporated in the electronic regulator unit of the turbomachine, controls the servo-valve 3a to servo-control the position of the slide of the metering unit to occupy a setpoint position that is determined as a function of the desired flow rate, with the actual position of the slide being detected by a sensor 3b. The high-pressure fuel delivered by the pump is also used as a servo-control hydraulic fluid for various pieces of equipment of the turbomachine, and in particular the servo-valve 3a that has a high-pressure port receiving the high-pressure fuel via a heater 7 and a low-pressure port connected to the low-pressure fuel circuit 1. The heater 7 is typically a fuel/oil heat exchanger. It serves to eliminate any icing that might be present in the fuel used for servo-control purposes and to cool the lubricating oil that is used in the turbomachine.
The outlet from the metering unit 3 is connected to the combustion chamber 8 that is fitted with injectors via a cut-off valve 9, e.g. controlled by a solenoid valve 9a. The cut-off valve 9 is a stopping and pressurizing cut-off valve that may possibly also be used for protecting the turbomachine against overspeeds. The solenoid valve 9a is controlled by the control unit 6 to close the cut-off valve 9 when required. A proximity sensor 9b provides the control unit 6 with information representative of the closed or open state of the cut-off valve. In a variant, a second bypass valve could be inserted between the metering unit and the stop cut-off valve, being controlled by an overspeed servo-valve or solenoid valve in order to divert the injected flow rate in the event of overspeed being detected.
Document WO 03/023208 describes a device for regulating the flow rate of fuel delivered to a gas turbine engine having a metering unit, a bypass valve maintaining a constant pressure difference between an inlet and an outlet of the metering unit, and a cut-off and pressurizing valve downstream from the metering unit.
Controlling the flow section of the metering unit that is subjected to a constant pressure difference enables the volume flow rate to be regulated.
In order to improve the accuracy of regulation while delivering a controlled mass flow rate, it is desirable to take account of the variation in the density of the fuel as a function of temperature. To this end, it is known to fit the slide of the metering unit with a rod made of a material that is selected to have a coefficient of thermal expansion that is suitable for compensating variation in the density of the fuel, with a small variation in the flow section thus being obtained as a function of temperature for a given detected position of the slide. It is also known from document WO 99/35385 to take account of variation in the density of the fuel as a function of temperature by using bimetallic disks having bearing thereagainst the rating spring of the bypass valve that is associated with the metering unit.
Furthermore, document US 2008/0163931 describes measuring the mass flow rate of fuel by means of a device that enables a measurement to be made of the flow section for fuel through the device as a function of a pressure drop through the device and from the density of the fuel. The measurement device is mounted at the outlet from a pump unit and the mass flow rate is regulated by controlling a variable-restriction valve by means of a servo-control loop that receives information representative of the desired mass flow rate and of the measured mass flow rate. A stop cut-off valve is also provided.